1. Field of the Invention
The present invention relates to a process for the dinitration of polyalkylbenzenes. Nitration is carried out in this case in a reaction medium of H.sub.2 SO.sub.4 with HNO.sub.3. No organic solvent is required for this process.
An important representative of dinitro-polyalkylbenzenes is 2,4-dinitro-1,3,5-triisopropylbenzene, which acts as an important precursor in plastics chemistry.
2. Description of the Related Art
1,3,5-triisopropylbenzene can be mononitrated in a simple manner in the system HNO.sub.3 /acetic anhydride; in a likewise facile reaction, 2,4,6-trinitro-1,3,5-triisopropylbenzene is obtained by addition of 1,3,5-triisopropylbenzene to a H.sub.2 SO.sub.4 /HNO.sub.3 mixture (J. Am. Chem. Soc. 65 (1943), 2434). A dinitro-triisopropylbenzene cannot be prepared by the process variants described. Other polyalkylbenzenes, such as 1,3-diisopropylbenzene, 1,4-diisopropylbenzene, 1,3-dimethyl-5-tert-butylbenzene and 1,3-dimethyl-5-dimethylpropylbenzene, can be nitrated by introducing each particular starting material together with concentrated sulphuric acid and adding concentrated HNO.sub.3 (J. Am. Chem. Soc., loc.cit.; German Offenlegungsschrift 3 317 649). In the case of dinitration, unsatisfactory results are obtained in this case.
The methods described are therefore suitable for the mononitration of polyalkylbenzenes. The trinitration mentioned of 1,3,5-triisopropylbenzene only succeeded as a result of high temperature elevation to approximately 100.degree. C., where the nitrated starting material melted and in the course of further nitration became a pasty mass. The yields achievable in the manner described and, particularly, the purities achievable are unsatisfactory. This must generally be assigned to the circumstances that a dinitro or trinitro product obtained in the further nitration encloses still unreacted mononitro product and thus prevents its further nitration.
Attempts have therefore already been made to carry out the nitration, for example of 1,3,5-triisopropylbenzene, in the two-phase system liquid/liquid (starting material in the organic solvent/acids) instead of in the two-phase system solid/liquid (mononitro-triisopropylbenzene/acids). Suitable solvents which have been described for this purpose are aliphatic hydrocarbons (mixtures having a boiling point range from 170.degree. to 190.degree. C.) (German Patent Specification 11 05 860). Such a procedure is always associated with the necessity for a solvent circulation and handling thereof; in addition, aspects relating to health at work and safety (fire hazard) must be taken into account.